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Heredity • Heredity is the passing on of characteristics from parents to offspring. These characteristics are called traits Gregor Mendel • It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity. • Mendel argued that parents pass on to their offspring factors (now called genes) that are responsible for inherited traits. • Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next. • A complete explanation requires the careful study of genetics—the branch of biology that studies heredity. Mendel chose his subject carefully • Mendel chose to use the garden pea in his experiments for several reasons. • Garden pea plants reproduce sexually, which means that they produce male and female sex cells, called gametes. Mendel chose his subject carefully • The male gamete forms in the pollen grain, which is produced in the male reproductive organ. • The female gamete forms in the female reproductive organ or ovary. Stigma Style Ovary Stamen Anther Filament Pistil • In a process called fertilization, the male gamete unites with the female gamete. • The resulting fertilized cell, called a zygote, then develops into a seed. • The transfer of pollen grains from a male reproductive organ to a female reproductive organ in a plant is called pollination. Stigma Style Ovary Stamen Anther Filament Pistil Mendel wanted to pick his own parent plants • When Mendel wanted to breed, or cross, one plant with another, he opened the petals of a flower and removed the male organs. Remove male parts • He then dusted the female organ with pollen from the plant he wished to cross it with. • The process of combining male cells from one plant with female cells from another is called cross-pollination. Pollen grains Transfer pollen Female part Male parts • He only wanted to study one trait at a time at first. • Mendel started by cross pollinating short pea plants with tall pea plants. • The only differing trait was height. • The tall pea plants he worked with were from populations of plants that had been tall for many generations and had always produced tall offspring. • They were said to be pure-bred tall • Likewise, the short plants he worked with were pure-bred for shortness. • A hybrid is the offspring of parents that have different forms of a trait, such as tall and short height. Monohybrid crosses • Mendel’s first experiments are called monohybrid crosses • Mono means “one” • The two parent plants differed from each other by a one trait— which in this case is height. The First Generation • He cross-pollinated the tall pea plant with pollen from a short pea plant. • All of the offspring grew to be as tall as the taller parent. The Second Generation • Mendel allowed the tall plants in this first generation to self-pollinate. • Three-fourths of the plants were as tall as the tall plants in the parent and first generations. The Second Generation • One-fourth of the offspring were as short as the short plants in the parent generation. P1 Short pea plant Tall pea plant F1 • In the second generation, tall and short plants occurred in a ratio of about three tall plants to one short plant.(3:1) All tall pea plants F2 3 tall: 1 short The Second Generation P1 • In every case, he found that one trait of a pair seemed to disappear in the F1 generation. • only to reappear unchanged in one-fourth of the F2 plants. Short pea plant Tall pea plant F1 All tall pea plants F2 3 tall: 1 short • Mendel concluded that each organism has two genes that control each of its traits. • We now know that these genes are located on chromosomes. • Genes exist in alternative forms. We call these different gene forms alleles. • An organism’s two alleles are located on their homologous chromosomes—one inherited from the female parent and one from the male parent. b B Bb Rule of Dominance • Mendel called the observed trait “dominant” and the trait that disappeared “recessive”. Bb • Mendel concluded that the allele for tall plants is dominant to the allele for short plants. • It is customary to use the same letter for different alleles of the same gene. T t for Tall for Short The letter “T” equals height. Short plant Tall plant t T T t T F1 Big T is Tall Little T is Short t All tall plants T t • An uppercase letter is used for the dominant allele and a lowercase letter for the recessive allele. Short plant Tall plant t T T t T • The dominant allele is always written first. Tt t F1 All tall plants T t The Law of Segregation • The law of segregation states: every individual has two alleles of each gene and when gametes (or sex cells) are produced, each gamete receives one of these alleles. Bb B b Tt Tt cross Law of segregation • Two organisms can look alike but have different underlying allele combinations. F1 Tall plant Tall plant T T t t F2 Tall Tall T T T Tall t 3 T Short t t t 1 Phenotypes and Genotypes • The way an organism looks and behaves is called its phenotype. • The allele combination an organism contains is known as its genotype. • An organism’s genotype can’t always be known by its phenotype. Bb BB • An organism is homozygous for a trait if its two alleles for the trait are the same. TT • The pure-bred tall plant that had two alleles for tallness (TT) would be homozygous for the trait of height. • An organism is heterozygous for a trait if its two alleles for the trait differ from each other. Tt • Therefore, the tall plant that had one allele for tallness and one allele for shortness (Tt) is heterozygous for the trait of height. What is the phenotype of the pea plant below? What is the genotype of the pea plant ? tt So therefore, what would you call this trait? Homozygous short Punnett Squares • In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross. • This method is called a Punnett square. Monohybrid cross • A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait. Heterozygous tall parent T T t t T T t T TT Tt t Tt tt t T t Heterozygous tall parent Probability • In reality you don’t get the exact ratio of results shown in the square. • That’s because, in some ways, genetics is like flipping a coin—it follows the rules of chance. Question 1 The passing on of characteristics from parents to offspring is __________. A. genetics B. heredity C. pollination D. allelic frequency The answer is B. Genetics is the branch of biology that studies heredity. Question 2 What are traits? Answer Traits are characteristics that are inherited. Height, hair color and eye color are examples of traits in humans. Question 3 Gametes are __________. A. male sex cells B. female sex cells C. both male and female sex cells D. fertilized cells that develop into adult organisms The answer is C. Organisms that reproduce sexually produce male and female sex cells, called gametes. Question 4 Which of the following genotypes represents a plant that is homozygous for height? A. Tt B. Hh C. tT D. tt The answer is D. An organism is homozygous for a trait if its two alleles for the trait are the same. It can be either homozygous dominant or homozygous recessive.